In optical systems such as a digital camera, a glass-made optical lens, in particular, an aspherical lens having such optical properties as a high refractive index and a low dispersion property is used. Currently, high-precision press-molding techniques using a molding tool are developed, and an aspherical lens is manufactured by a high-precision press-molding method.
As the high-precision press-molding method, there is a reheat press method in which a preform having a predetermined mass and shape is obtained from a glass molten liquid with a dropping, and the obtained preform is put into a mold, followed by reheating and press-molding.
In order to improve the productivity of the reheat press method, a high-precision preform is necessary. In the case where a preform which has no devitrification and striae and has a predetermined mass is subject to the high-precision press-molding, a polishing step is unnecessary. On the other hand, it is preferable that a preform has a low glass transition temperature (Tg) from the viewpoint of improving the durability of a mold.
For example, a preform is manufactured by melting glass raw materials in a platinum tank or the like at a high temperature; lowering a molten liquid temperature to cause the molten liquid to flow through a pipe; dropping the molten liquid from a platinum nozzle; and cooling the molten liquid in a receiving mold. The molten liquid temperature in the pipe is held at an approximately liquidus temperature (TL) from the viewpoints of improving productivity and preventing devitrification. The liquidus temperature TL described herein represents the minimum temperature at which crystals do not precipitate in the molten liquid when the glass molten liquid is held. In the case where crystals precipitate in the molten liquid, the glass is devitrified during the manufacture of a preform, which causes appearance defects. Therefore, at the time of manufacturing a preform, the molten liquid temperature is held to be high to the extent that devitrification does not occur.
Meanwhile, in the case where the molten liquid temperature is high, components are likely to volatilize from the molten liquid surface during the dropping of the molten liquid from the nozzle and during cooling in the receiving mold. In the case where the components volatilize, the glass composition on the molten liquid surface becomes nonuniform and striae are generated on the glass surface, which causes appearance defects. Therefore, during preform-molding, the molten liquid temperature is held to be low to the extent that the components do not volatilize.
As the composition of an optical glass having a high refractive index and a low dispersion property, a glass containing B2O3—La2O3 as a main component is widely used. Since B2O3—La2O3-based glass has a high glass transition temperature Tg, the glass transition temperature Tg is reduced by adding alkali components such as Li2O. However, in the case where an alkali component is added, a refractive index nd is decreased. Therefore, there is a concern that the desired optical constants may not be obtained.
Patent Documents 1 and 2 disclose B2O3—La2O3-based glasses containing 0.2 mass % to 3 mass % of Li2O; and a total content (La2O3+Gd2O3+Y2O3) of 35 mass % to 60 mass % of La2O3, Gd2O3, and Y2O3, in which a glass transition temperature Tg is 630° C. or lower and is suitable for press-molding. These glasses have optical constants of a refractive index nd of 1.82 to 1.86 and an Abbe's number υd of 37 to 44.
However, these glasses have a high liquidus temperature TL of 1000° C. or higher. Therefore, in the case where the molten liquid is held at the liquidus temperature TL or higher, B2O3 and an alkali component volatilize and there is a concern that surface striae may be generated at the time of preform-molding.
On the other hand, a manufacturing method, in which the generation of striae on a preform surface is suppressed even when a liquidus temperature TL is high and a molten liquid temperature is high, is known. Patent Document 3 discloses a method in which, when the molten liquid is dropped during preform-molding, gas is blown toward the molten liquid to rapidly lower a surface temperature and thus to suppress the volatilization of components. According to this method, the volatilization of components can be suppressed and surface striae can be prevented; however, most of gas blowing conditions are obtained from experience and the reproducibility is low.
[Patent Document 1] JP-A-2011-6318
[Patent Document 2] WO 2009/72335
[Patent Document 3] JP-A-2009-263228